The present invention relates to an aluminum coated steel sheet having formability and corrosion resistance and to a process for the production thereof.
Aluminum hot dipped steel sheet products, prepared using a practically 100% aluminum hot dipping bath, have satisfactory weather and corrosion resistances. They pose, however, a problem in their formability owing to the presence of a relatively thick (e.g. about 20 .mu.m) intermediate layer of intermetallic compounds formed between the steel substrate and aluminum coating layer. They have a drawback in that when bent, pressed, drawn or otherwise mechanically worked even at a slight working rate, the intermediate layer often cracks and the coating layer or layers frequently peel off. For this reason, it has become the practice to add silicon to an aluminum hot dipping bath thereby to control the growth of the intermediate layer of intermetallic compounds to a thickness of about 2 to 4 .mu.m. The product, Al--Si hot dipped steel sheet, having a good formability as well as excellent heat and corrosion resistance, is widely used for various applications.
With such an Al--Si hot dipped steel sheet, there still remains a problem in that when worked at a severe working rate, the Al--Si coating layer or layers often readily crack, and pits of red rust appear relatively early and develop in those areas of the steel substrate where the coating layer or layers have cracked. This is partly because the Al--Si coating layer has a cast structure of an insufficient elongation, and partly because the continuous intermediate layer, essentially consisting of Al--Fe--Si intermetallic compounds and having a thickness of about 2.0 to 4.0 .mu.m, often locally cracks at the time of working, leading to localized concentration of internal stress in the coating layer.